1. Field of the Invention
The present invention relates to a multilayer ceramic copper circuit board, more particularly to a circuit board comprising a glass/ceramic composite having a low dielectric constant and a low thermal expansion coefficient, and copper conductors having a low electrical resistivity.
2. Description of the Related Art
A multilayer circuit board comprises layers of electrical insulators, usually ceramic, and metallic conductors. The ceramic must have a low dielectric constant, to enable a high-speed transmission of signals, and a thermal expansion coefficient nearly as low as that of silicon chips mounted on the ceramic board, to withstand temperature changes during the operation, as well as other desirable electrical and mechanical properties.
Previously, a multilayer circuit board was produced from a high melting point metal, e.g., molybdenum or tungsten, and a ceramic, e.g., a sintered alumina Al.sub.2 O.sub.3. However, alumina has a high dielectric constant of 9.9 and a high thermal expansion coefficient of 6.8.times.10.sup.-6 /.degree. C., and thus it is well known to replace the alumina with other oxides, e.g., silica, quartz glass, and mullite. Mullite 3Al.sub.2 O.sub.3. SiO.sub.2 has a dielectric constant of 6.5 and a thermal expansion coefficient of 4.0.times.10.sup.-6 /.degree. C., which is nearly equal to the 3.5.times.10.sup.-6 /.degree. C. of a silicon chip mounted on the ceramic board.
Recently, copper has been used for the conductors, because of the low electrical resistivity and price thereof. In this case, during the firing of the multilayer board, the copper must not be oxidized, but the organic binder included in the ceramic compositions must be eliminated without leaving residual carbon. If the matrix glass softens before the binder is completely burnt out, residual carbon is trapped in closed holes in which water vapor is present. The carbon then reacts with the water to generate carbon dioxide gas. This causes a bloating of the glass/ceramics composites, and reduces the mechanical strength thereof.
To avoid such defects, thermally depolymerizable resins are commonly used as the binder of the ceramic composition, since they can be thermally decomposed into low-molecular weight species, and thus are easily removed at a relatively low firing temperature.
Borosilicate glass has a dielectric constant of 4.0 to 4.9 and a thermal expansion coefficient of 3.2 to 4.6.times.10.sup.-6 /.degree. C. and, therefore, is commonly used as the matrix of the glass/ceramic composite structure. However, borosilicate glass has a tendency to precipitate cristobalite SiO.sub.2 when the glass is fired at a temperature below the melting point of copper. Cristobalite has a thermal expansion coefficient of about 50.times.10.sup.-6 /.degree. C., which is more than ten times the 3.5.times.10.sup.-6 /.degree. C. of silicon, and thus causes damage to the silicon semiconductor devices due to thermal changes during the operation.
It is known that alumina can prevent the precipitation of cristobalite from borosilicate glass, and that quartz glass SiO.sub.2 has a low dielectric constant of 3.8, which will compensate the high dielectric constant of 9.9 of alumina.
In U.S. Pat. No. 4,642,148, K. Kurihara et al to Fujitsu disclose a method for producing a multilayer ceramic circuit board including the steps of forming a multilayer structure consisting of patterns of copper-based paste and glass/ceramic composite layers, the glass/ceramic composite layers consisting of a mixture of 10 percent to 75 percent by weight of .alpha.-alumina, 20 percent to 60 percent by weight of crystallizable or noncrystallizable glass which can be sintered at a temperature lower than the melting point of copper, and 5 percent to 70 percent by weight of quartz glass, based on the total weight of the glass/ceramic composite, blended with a binder containing a thermally depolymerizable resin; prefiring the multilayer structure in an inert atmosphere containing water vapor, the partial pressure of which is 0.005 to 0.3 atmosphere, at a temperature at which the thermally depolymerizable resin is eliminated; and firing the multilayer structure in an inert atmosphere containing no water vapor at a temperature below the melting point of copper so as to sinter the glass/ceramic composite. The prefiring may comprise a first prefiring step at 350.degree. C. to 450.degree. C. and a second prefiring step at 650.degree. C. to 900.degree. C., and the firing is carried out at a temperature higher than 900.degree. C. and lower than 1083.degree. C.
In U.S. Pat. No. 4,654,095, to du Pont, referred to as the basis of a priority application of Japanese Unexamined Patent Publication No. 61-220203 laid open on Sept. 30, 1986, J. I. Steinberg discloses a dielectric composition, which comprises 50% to 75% by weight of a noncrystallizable glass having a softening temperature of 630.degree. C. to 700.degree. C., the difference in the softening temperature and the deforming temperature being 50.degree. to 70.degree. C., and 50% to 25% by weight of a refractory, which is substantially insoluble in the glass at 825.degree. to 900.degree. C. The refractory and glass solids may be selected from Al.sub.2 O.sub.3, mullite, cordierite, SiO.sub.2, CaZrO.sub.3, forsterite, ZrO and mixtures thereof.
However, in all of the Examples, Steinberg teaches that a refractory oxide or oxides were sintered with a lead aluminum silicate type glass, and in Example 3, that mullite was used as the refractory to form ceramic green tapes, which were laminated and fired at 350.degree. C. for 40 minutes, and subjected to a 90 minute heating cycle with a peak temperature of 850.degree. C. for 15 minutes in air, and that the fired ceramic exhibited practically no deformation and had a dielectric constant of 6.5 at 1 kHz. Steinberg also teaches in Example 4 that a lead aluminum barium silicate glass having a softening temperature of 715.degree. C. produced severely bowed sintered laminated parts.
In U.S. Pat. No. 4,655,864 to du Pont, referred to as the basis of a priority application of Japanese Unexamined Patent Publication No. 61-220204 laid open on Sept. 30, 1986, J. R. Rellick discloses a dielectric composition which comprises 40% to 70% by volume of a noncrystallizable glass having a softening temperature of at least 500.degree. C., and a viscosity of 1 .times.10.sup.6 poises or less at 825.degree. to 1025.degree. C.; and 60% to 30% by volume of a mixture of refractory oxides comprising 1% to 59% by volume of Al.sub.2 O.sub.3, and 59% to 1% by volume of a secondary refractory selected from .alpha.-quartz, CaZrO.sub.3, and fused silica, which is 20% by volume, on the basis of the total inorganic solids.
Rellick teaches in Example 1 that a ceramic green tape which comprises a lead calcium aluminum silicate glass, quartz and alumina was laminated on an alumina substrate with a prefired copper paste printed thereon, and was subjected to cycle firing for 1 hour with a peak temperature of 900.degree. C. for 10 minutes in nitrogen, and that the obtained ceramic exhibited a dielectric constant of 6.5 to 7.5 at 1 kHz and no deformation.
It is noted that Steinberg teaches the use of mullite and SiO.sub.2 with a noncrystallizable glass, which has a softening temperature in the range of 630.degree. to 700.degree. C., and that Rellick teaches the use of a mixture of refractory oxides, i.e., Al.sub.2 O.sub.3, as an essential component of a first refractory and second refractories which may be fused silica and mullite with a noncrystallizable glass, which has a softening temperature of at least 500.degree. C., but a glass having a higher softening temperature is not discussed.